如何从数据库中删除SQLAlchemy多对多孤儿？

from sqlalchemy import create_engine, Column, Integer, String, Table, ForeignKey
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

# 创建数据库连接
engine = create_engine('数据库连接字符串')
Base = declarative_base()

# 定义多对多关系的中间表
association_table = Table('association', Base.metadata,
    Column('left_id', Integer, ForeignKey('left.id')),
    Column('right_id', Integer, ForeignKey('right.id'))
)

# 定义左侧实体类
class Left(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    rights = relationship("Right", secondary=association_table, back_populates="lefts")

# 定义右侧实体类
class Right(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    lefts = relationship("Left", secondary=association_table, back_populates="rights")

# 创建数据库表
Base.metadata.create_all(engine)

# 创建会话
Session = sessionmaker(bind=engine)
session = Session()

# 查询符合条件的多对多孤儿数据
orphans = session.query(Left).filter(Left.rights == None).all()

# 删除多对多孤儿数据
for orphan in orphans:
    session.delete(orphan)

# 提交事务
session.commit()